The present invention concerns the condensing of vapour phase compounds or elements, typically metals such as magnesium, obtained by reduction processes. These include metallothermic and carbothermic processes. The invention in particular concerns a process and apparatus for condensing and collecting metal and other vapours by the use of an expansion nozzle.
Magnesium extraction from its mineral ores has been the subject of scientific and technical studies over more than a hundred years. Magnesium metal extraction has drawn particular interest and effort due to this metal's material properties as an important alloying element in aluminium and other metals. Furthermore in recent years, magnesium has become important as a lightweight, yet strong structural material in its own right, particularly in the automobile industry. The method of extraction has followed two lines, i.e. electrolytic reduction of water-free molten salts, or pyro-metallurgical routes involving the reduction of oxide and carbonate forms of the metal, using carbon or metal reduction agents.
The main technical problems in magnesium metal manufacture in general are not only related to the need for continous high energy inputs due to the metal's inherently strong negative electrode potential. For the pyro-metallurgical routes there is additionally the necessity of a high reaction temperatures to initiate and maintain the reduction process, which however can be obtained with appropriate choice of furnace type. In the pyro-metallurgical routes, there are two categories of reductants: carbon (in carbothermic reduction) and certain metals (in metallothermic reduction). In the high temperatures regimes employed in both cases, the reduced metal will appear in gaseous form, either alone as in metallothermic processes, or together with carbon monoxide in carbothermic reductions. Typical reducing agents are solid, liquid or gaseous forms of other metals, carbon, hydrocarbons or other organically derived materials, and hydrogen. When the reduced metal coexists with the oxide form of the reductant at high temperatures, it can only be stabilised in metal form at lower temperatures when it is cooled very fast to below its melting point.
An inherent problem of cooling a hot gas containing both the reduced gas in metallic form, and the oxide form of the reductant, is that the gas mix on cooling reverses the reaction (back reaction) so that the resulting product can be wholly or partly reverted to metal oxide and the elemental reductant. For example, if carbon is used as the reductant, the primary reduction reaction is given by:C(s)+MgO(s)→CO(g)+Mg(g)  Eq.[1]
This reaction is favourable in the temperature range of 1600 to 1900° C., depending on total pressure in the gas; it is valid at the lower end of the temperature range by reducing the pressure of the gas through evacuation, or through the addition of appropriately heated inert gas.
Upon cooling of the gas, the following reaction occurs in whole or in part:CO(g)+Mg(g)→C(s)+MgO(s)  Eq.[2]
Since any chemical reaction takes time, condensing systems for this type of metallurgical processing rely on swift or “instant” cooling so that back reactions are reduced to a minimum. To achieve swift cooling of a gas several methods are known in the art; however, the present invention preferably makes use of a device known as de Lavalle adiabatic nozzle, schematically depicted in FIG. 6 hereinafter.
Passing the hot reaction reaction gasses through a nozzle as depicted in FIG. 6, rapid cooling can be achieved as indicated in Table 1 below. The gases are accelerated to the speed of sound as they pass through the nozzle. The temperature of the gas drops from reaction temperatures to a temperature determined by the pressure differential across the nozzle and its geometry, as known in the art. This cooling occurs in the residence time indicated in the third column in Table 1 for various length nozzles.
TABLE 1Residence Times of Gases in a Nozzle of Different LengthsNozzle neckGas speedResidence timelength (cm)m/sin seconds1997.21.00282E−052997.22.00563E−055997.25.01408E−056997.26.01689E−0510997.20.00010028215997.20.00015042220997.20.000200563Cp/Cv = 5/3 for monoatomic gas (Mg)Cp/Cv = 7/5 for di-atomic gas (CO)Gamma = Cp/CvSpeed of Sound = (gamma * R/nT)1/2, where R is the gas constant, and T is the temperature in degrees Kelvin.
U.S. Pat. No. 3,761,248 discloses the metallothermic production of magnesium which involves the condensation of magnesium vapour evolved from a furnace in a condenser. The condensation is promoted using a flowing inert gas to draw the vapour into the condenser.
WO 03/048398 discloses a method and apparatus for condensing magnesium vapours in which a stream of vapour is directed into a condenser which has a lower crucible section from which liquid magnesium may be tapped. A molten lead jacket is used to cool the crucible section.
US application 2008/0115626 discloses the condensation of magnesium vapour in a sealed system in which liquid metal is continuously tapped from a crucible portion.
U.S. Pat. No. 5,803,947 discloses a method for producing magnesium and magnesium oxide. A condenser for the collection of magnesium liquid is fed via a converging/divergent nozzle for supersonic adiabatic cooling of the gas passing through the nozzle. No details are given of the structure or configuration of the nozzle and condenser, although it is stated that a cyclone is used to precipitate particles entrained in a carrier gas downstream of the nozzle.
Descriptions of adiabatic cooling systems per se are known; vide e.g. “Compressible Fluid Flow” Authored by Patrick H. Oosthuizen et al., 1997, ISBN 0-07-048197-0, McGraw-Hill Publishers.
U.S. Pat. No. 4,488,904 discloses a method in which metallic vapour (such as magnesium) is directed through a convergent-divergent nozzle which cools the metal to a level at which oxidation will not take place. The metallic vapour is directly or indirectly led onto a metal retrieving pool which, in the case of magnesium collection, comprises molten lead, bismuth, tin, antimony or a mixture thereof. EP-A-0 124 65 similarly discloses a method for collecting liquid metal (magnesium) from vapour via an adiabatic nozzle. In this document the vapour is collected in a pool of molten magnesium.
JP-A-63125627 discloses a method of forming metal matrix composite material in which a metal vapour is directed through an adiabatic nozzle. A reactive gas is introduced into the nozzle so as to react with the metal and form particulate metal compound. The compound is directed from the nozzle into a metal pool of the metal matrix material. Hence a dispersion of metal compound particles in a metal matrix is formed.
U.S. Pat. No. 4,147,534 discloses a method for the production of Magnesium (or Calcium) in which a metal vapour is passed through an adiabatic nozzle and directed onto a cooled surface, which may be a rotating cylindrical surface in one embodiment. The solidified magnesium particles are scraped from the surface and fall into a screw conveyor which leads to a furnace for melting the particles. The molten magnesium then falls into a collection reservoir.
JP-A-62099423 discloses apparatus for collecting metal vapour directed from an adiabatic valve. A collection pool is provided with a perforated tray or grid over which molten metal is circulated so as to collect metal vapour and reflect oxidizing gas.
Problems arise in the prior art processes in several areas. One is the oxidation or contamination of the condensed droplets or particles in the condensing chamber. Another is oxidation or contamination of the liquid metal collected from the nozzle, in both cases due to carrier or reaction gasses present in the condensing chamber.
Another problem concerns the efficient adsorption of the particles or droplets into bulk liquid when at the localised region of the liquid in which the beam of condensed droplets or particles impinges.
The present invention its various aspects seeks to solve one or more of the above problems in one or more ways. The solutions and other benefits of the invention will be evident to the skilled person from the following description of the invention.